In order to build the editor you need to install all dependencies first. To do it, open the terminal in the project directory and type:
npm install
Make sure that you have the node and npm installed first. If not, then follow the instructions on the Node.js documentation page.
If you need to add additional CKEditor plugins, you can install them in the build. Just follow the Adding a plugin to an editor tutorial
If you need to build your own plugins check CKEditor's documentation or src/plugins/ for examples.
If you have already done the Installation and Adding or removing plugins steps, you're ready to rebuild the editor by running the following command:
npm run build
This will build the CKEditor 5 to the build directory.
In order to update iTop CKEditor's build you need to push your changes to github. Then run the following command in iTop's root directory:
npm install https://github.com/Combodo/ckeditor5-itop-build.git --omit=dev